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Chemistry 11 Notes for Chemistry Notes

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Structure Of Benzene

Discussion
Here we are going to discuss why benzene does not have an open chain structure. In the coming lines we will cover different possibilities related to the structure of benzene molecule.

Structure of benzene is a controversial topic because benzene has outstanding characteristics. It behaves both as saturated and unsaturated hydrocarbon. Here we discuss different aspects of the structure of benzene.

Let us assume that benzene belongs to alkane family
In case of an alkane, molecular formula of benzene must be the according to CnH2n+2 i.e. C6H14. But there are only six H-atoms in benzene molecule. This shows that benzene does not belong to alkane family.

Let us assume that benzene belongs to alkene family
In case of an alkene, molecular formula of benzene must follow CnH2n i.e. C6H12. But there are six H-atoms in benzene instead of 12. This shows that benzene is not a member of alkene family.

Let us assume that benzene belongs to alkyne family
In case of alkyne, molecular formula of benzene must be according to CnH2n-2 i.e. C6H10. This shows that for six C-atoms there must be 10 H-atoms and benzene contains only six H-atoms. This concludes that benzene is not an alkyne.

Let benzene belongs to open chain
Hydrocarbons containing multiple bond
If we assume that benzene belongs to any hydrocarbon which contains multiple bonds then we can draw some Possible structures of C6H6 molecule like the one:
CH2=CH-CH=C=C=CH2
But this type of compound is a highly unsaturated compound and it must give addition reactions under ordinary conditions. Now we discuss benzene in the light of reactions.

Structure of benzene in the light of chemical reactions
If we compare the reactions of benzene with the reactions of any aliphatic hydrocarbon (alkene or alkyne) it must give the following type of reactions.
C6H6 + X2 ® C6H6X2 + C6H6X4 + C6H6X6 (all are addition reactions)
But in actual practice this reaction is not observed. However, benzene gives the following type of reactions:
C6H6 + X2 ®C6H5X + HX
Which is a substitution reaction. This shows that benzene does not belong to aliphatic hydrocarbon but it must be a special type of hydrocarbon.
It should be noted down that benzene gives addition reactions but under extraordinary conditions like
C6H6 + 3H2 ® C6H12 (Cyclo Hexane)
C6H6 + 3Cl2 ® C6H6Cl6 (Hexa Chloro Cyclo Hexane)
When benzene is treated with Cl2, Br2 in dark or with aqueous KMnO4 or with dilute acids, no reaction occurs.

Conclusion
From the above arguments it is clear that benzene is not an open chain hydrocarbon.

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